1.4 TSI random misfire

[Paul_OH]

I've had a random misfire for the last 6-8 weeks which is starting to do my head in.
It's only present on really hot days (35 degrees+) and even then not every time and sometimes not even within the same drive. It's most noticeable at low revs, high load and the best way for me to replicate is hold 5th gear at 70km/h at full throttle (approx 1800-2000rpm). Once past 3500-4000rpm it is gone.
The best way to describe it is similar to traction control kicking in combined with wheel hop.

I've had it in at VW twice and at a VW specialist once and have so far replaced the N249 valve (latest revision with a piston), N75 valve and new spark plugs.

I have it booked in again with VW later in the month for the foreman to take it home overnight but unless it's a hot day the problem likely won't be present. Unfortunately it's not leaving any error codes for my Vag-com or VW's scan tool to find. The dealer has also visually checked the coil packs for arcing however none was found and with it being so random they discounted this (fyi, I have absolutely NO experience with coil packs).

Any ideas for additional things to check would be appreciated.

Paul

[James Roxas]

Hi Paul,

I had this problem late last year.
At first my TSi Jetta was misfiring and resulting in the engine light flashing.
I took it to the closest VW dealer which they made me pay for new spark plugs and let's just say that was a temporary fix.

One month down the track sure enough it started misfiring again. This time I took it to a different VW service center as my car is still under warranty. They changed the coil packs and flashed the ecu and since then it's been fine (all covered by warranty).

It did misfire once more a few weeks later, I took it back to have it checked out straight away and apparently the error code showed it was due to low fuel? It hasn't happen since even when the fuel light is on.

So it may be worth checking out the coil packs and flashing the ecu from my experience.

[Paul_OH]

Thanks James,
Car is with VW at the moment (today & tomorrow) so will get them to check out the coilpacks too although I have already requested that.
Plugs were changed in January (with no result) and coilpacks were checked for evidence of arcing but that was all ( I have no idea what sort of checks are done on coilpacks?!). They checked the ECU version in February and it was the latest version at the time - will ask if it can be re-flashed though.

Thanks for the reply

[Paul_OH]

Problem now appears to be solved and traced back to a cracked insulator on #2 spark plug. These weren't checked previously as they were only replaced in January, as part of the original troubleshooting by my specialist........ :duh: